Many types of electrostatic filters have been proposed for removing small particulate materials such as dust, smoke and the like from gases such as air or the exhaust gases of vehicles or industrial processes. Typically, such filters rely in one way or another on the ionization of the particulate material by friction or by a high-voltage electric field, so that they may be trapped and held by electrostatic forces. A common disadvantage of ionizing electrostatic filters is that they operate at sufficiently high voltages to require expensive insulation and safety precautions as well as substantial power and that they produce ozone which constitutes a health hazard.
Non-ionizing electrostatic filters have also been proposed in the past, but their use tends to be limited to special situations, such as the capture of partially conductive soot particles from diesel exhaust.
Mechanical filters (including high-efficiency particulate air (HEPA) and ultra-low penetration air (ULPA) filters) not using electric fields are also common, but they are basically unable to capture particles smaller than their pore size, and they are also subject to rather rapid clogging by captured particles. The clogging takes place mostly on the inflow surface of the filter, and the thickness of the filter material for holding particles is not utilized.